This invention relates to an audio amplifier and more particularly, to an audio amplifier that enjoys an improved performance when compared to conventional amplifiers, due to an improved linearity in various amplifying stages and a reduction in parisitic interactions.
Some designers of current state-of-the-art audio amplifying equipment are becoming aware of a general lack of correlation between the subjective quality of the sonic performance of such equipment and the objective quality as measured by laboratory measuring instruments. For some years amplifying equipment for high fidelity applications has been designed around its laboratory performance based on a small number of classical steady state tests. One popular test which has gained wide acceptance because of the simplicity involved is the total harmonic distortion test. This test checks for modifications done to a single frequency in the equipment under test. An actual music signal is composed of a large number of frequencies however, and equipment that has been designed to pass a single frequency or a simple combination of frequencies under steady state conditions may not pass a complex, constantly changing wave form with good fidelity.
The traditional method of designing audio equipment involves connecting a number of high gain amplifying stages together and applying a negative feedback loop around the entire number of stages to produce the desired amplifier gain and total harmonic distortion characteristics. Total harmonic distortion figures measured in the laboratory are generally lowest and thus best for those amplifier designs having the largest amount of feedback. The gain stages in most of these types of amplifiers are designed for maximum gain with little attention paid to the linearity of a single isolated stage.
The problems associated with these types of amplifiers are numerous. For example, the subjective quality of performance is generally poor. Also, the phase shift within the amplifier reacts with feedback and causes oscillation. In addition, a form of time delay distortion is produced when a correction signal is fed back through the feedback loop and amplifier after the signal that was to be corrected had already passed out of the amplifier. Transient intermodulation distortion is produced when fast rising waveforms interact with phase compensating networks and high gain--high feedback situations to completely saturate the input stages of the amplifier. Distortion produced from amplifier clipping is much more apparent in a high feedback amplifier for two reasons. First, the electronic devices that make up the amplifier are driven into heavy saturation for which recovery takes a relatively long time. Secondly, the clipping is sharper and more higher order harmonics are generated resulting in a harsher and more obvious clipping sound than would otherwise be generated.
Although prior designs exist which do not use a large negative feedback loop, they are not without problems. For example, the active devices that make up the amplifying stages are somewhat nonlinear in their transfer characteristics thus producing unacceptable levels of harmonic distortion. Also, there are parisitic capacitances between the input and output ports on these active devices which restrict the frequency response of the amplifier and cause higher loading of the source at high frequencies. In addition, the DC voltage levels can drift due to temperature changes and changes in component values thus restricting the useful dynamic range of the amplifier.